Housing and welfare in laboratory rats: time-budgeting and pathophysiology in single-sex groups

Alderley Park (Wistar-derived) rats,Rattus norvegicus, were maintained in single-sex groups of eight for 9 weeks under open-room conditions. Both sexes could be categorized into more or less discrete social classes based on the directionality of aggression within dyads. The difference in the number of aggressive acts initiated and received (agonistic score) provided a measure of social pressure for each rat. Males and females differed in the relationship between social class and social pressure and thus in the behavioural and pathophysiological effects of social pressure. Differences in the relationship both between and within sexes can be interpreted in terms of the degree of frustration of different social strategies. The effect of social pressure on behaviour was most obvious in time spent sleeping, exploring and attempts to escape; dominant individuals slept more and low status individuals, especially females, spent more time moving around the enclosure and stretching up the walls. Individual and sex differences in social strategy and time budgeting were reflected in levels of hormone and antibody concentrations during the period of grouping, and in changes in organ histology suggestive of early pathology. Corticosterone (but not testosterone) concentrations correlated significantly with aggressive behaviour, but with aggression subsequent to, rather than preceding or concurrent with, blood samples. Individual differences in aggression initiated and received during introductions did not reflect those recorded in established groups of either sex. The results are discussed in the light of other studies of relationships between social strategy and pathophysiology in rats and the selection pressures likely to be acting on free-living animals leading to a new interpretation of the welfare implications of previous work.